Cooling arrangement for high frequency low pass filters



Feb. 11, 1969 F. A. DENES 3,427,577

LING ARRANGEMENT FOR HIGH FREQUENCY LOW PASS FILTERS Filed June 5. 1966 FIG. 1. (PRIORART) 22 FIG. 2a.

2? ,N0N'-METALL1Q EPOXY ADHESIVE. 21 22 INVENTOR PETER A.DENES ATTORNEY United States Patent 3,427,577 COOLING ARRANGEMENT FOR HIGH FRE- QUENCY LOW -PASS FILTERS Peter A. Denes, 9101 Crestwood NE., Albuquerque, N. Mex. 87112 Filed June 3, 1966, Ser. No. 555,047 US. Cl. 336-61 Int. Cl. H011 27/08; H01h 7/08; H01b 7/34 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to cooling arrangements for electrical apparatus and more particularly to an improved and more efiicient arrangement for dissipating heat generated in an electrical device while also affecting the functional performance of the electrical device.

The present invention relates more specifically to heat transfer problems found in high frequency low pass filters which are capable of withstanding very high high frequency input voltages. Such filters must have a high input impedance to limit the high frequency input power in the filter. Hence, the first component of the filter must be an inductor, and actually, the impedance of this inductor alone limits the input high frequency current of the filter, because the next component of the filter is a capacitor to the ground. In such filters, the requirement often is that the first inductor should have a rather low Q value so that the high power unwanted signal should not just be reflected by the filter but a high percentage of that signal should be consumed in the first inductor. In such cases, the dissipation of the heat generated in this first inductor is a problem. Since the core is made of a ferrite material which does not have a high Curie point, the filter would cease to work if the hottest point of the first inductor core reaches its Curie point and loses its ferromagnetic qualities.

It has been proposed in the prior are to use so-called potted cores in the first inductor, the outside surface of which is coated with a metal layer and this metal layer is soldered directly to the metal case of the filter. This design minimizes the temperature difference between the outside surface of the inductor and the metal case of the filter. On the other hand, potted cores have a very poor temperature distribution in their inside. Their central part in which most of the heat is generated is a long rod of relatively small cross-section. Many of the potted cores used as firs-t inductors in such filters are made of ferrites which have a mediocre heat conductivity and a rather low Curie point. In actual applications, the middle part of the central rod in a potted core can be 100 C. hotter than its outside surface and consequently the temperature of the middle part would get near to the Curie point, even if the outside surface is adequately cooled by cooling the case of the filter.

Toroids as inductor configurations have a much better heat transfer pattern. They have a large cross-section and a small length to conduct to the outer periphery the heat generated in them. On the other hand, there are wires wound on their outer surface and it therefore cannot be metallized and soldered to the case of the filter.

Consideration has been given to the application of an electrically insulating but thermally conductive filling between the outer surface of the inductor and the inside surface of the case. Such relatively better heat conductor but electrical insulator could be a plastic resin filled with beryllia powder. The heat conductivity of such systems still would be less than 1% of that of copper. Another disadvantage of such filling would be that it would increase the parallel capacitance of the winding of the first inductor of the filter to ground and, consequently, at higher frequencies the input impedance of the filter would be quite low and would not be able to limit the input high frequency current to the acceptable value.

Accordingly, an object of the present invention is to secure maximum heat conductance between the outside periphery of the inductor and the case of the filter.

It is also an object of the present invention to minimize the capacitance between the winding of the inductor and the case of the filter.

Still another object of the present invention is to minimize the temperature difference between the hottest point of the inductor and its outer surface.

These as well as further objects and advantages which are inherent in the invention will become apparent from the following description, reference being had to the accompanying drawing wherein:

FIG. 1 is a cross-sectioned view of a potted core inductor of a prior art filter;

FIG. 2 is a cross-sectioned view of a filter of the present invention;

FIG. 2a is an enlarged section of FIG. 2; and

FIG. 3 is a cross-sectional view of another embodiment ment of a filter of the present invention.

FIG. 1 illustrates in a cross-sectioned view a potted core made up of two halve sections 11 and 12. There is an airgap between sections 11 and 12. The airgap can be minimized by grinding, but even then it lowers considerably the permeability and the magnetic losses, both effects considered undesirable for this application. These effects are eliminated in toroid-shaped inductors. The outside surface of the potted core is a metallized layer 13 and it is soldered to the case 14 of the filter. The hottest point in the inductor is the middle of stud 15. As discussed above, stud 15 has a small cross-section and long length. Measurement has shown that the middle part of stud 15 can be C. warmer than the metallized outside surface 13 of the core. The wiring 16 is surrounded by air and thus it has a relatively small parallel capacitance to the case 14.

An example of toroid shaped inductors built in accordance with the present invention is found in one embodiment illustrated in FIGS. 2 and 2a. In the construction shown in those figures the toroidal core 21 is surrounded by the wiring 22. The wiring occupies a part of the outside circumference of the toroid while wired in one layer. The heat transfer element 26 is a copper sheet bent in an undulating way and is placed between the core 21 and the case 25. The heat transfer element 26 is attached to the ferrite core 21 by a thin layer of epoxy adhesive 23 and to the case 25 with a silver filled epoxy resin 24. This arrangement also has the advantage that between the case 25 and core 21 there is a somewhat resilient connection through the undulated heat transfer element 26 and shocks, vibrations, and other mechanical effects, on the case 25 reach the fragile ferrite core 21 to a highly reduced degree.

Basically, the present invention is an input inductor of a high power loaded high frequency filter having a magnetic core 21 of a heat conductance pattern characterized by a large cross-section and short length of the heat transfer path between the hottest point of the inductor and its outer surface, and having a heat transfer element 26 being in touch with a portion, approximately at least 5%, of the outside surface of the magnetic core 21 of the inductor and made of a material of high heat conductivity, said heat transfer element being in touch both with a part of the outside surface of the magnetic core 21 of the inductor and a part of the inside surface of the case 25 of the filter in a low temperature drop connection.

In this specification, the expression low temperature drop connection means a contact between the heat transfer element 26 and the core or case 25, respectively, through which no more than approximately of the total temperature drop occurs. Such a contact can be achieved by a spring loaded touch between the heat transfer element 26 and the core or case 25, or, a very thin layer of adhesive resin can be applied between the adjacent surfaces. If the thickness of the adhesive material is less than 0.002, the temperature drop through the adhesive material is quite small even if its heat conductivity is low. The heat conductivity of the adhesive resin can be considerably increased by filling it with high heat conductivity particles such as silver, copper, magnesia, beryllia, etc. Beryllia filled adhesive is recommended on the surface of the inductor core 21, beryllia being an electrical insulator; while metal filled adhesive can be applied between the heat transfer element 26 and the case of the filter. Naturally, both the heat transfer element 26 and the case 25 being metals, they can also be soldered together.

The heat transfer element 26 is preferably made of copper or silver or of another suitable metal or alloy having a high heat conductivity. As defined, it is in a low temperature drop connection with the outer periphery of the inductor core 21. As ferrites have an average of about 5% of the heat conductivity of copper, if the heat transfer element 26 touches only 5% of the outer surface of the ferrite core 21, the heat conductance pattern in the heat transfer element 26 would still be equal to that in the ferrite core 21 per length unity.

The heat conductance in ferrite toroid shaped cores is quite good. Generally, the toroids have an approximately twice as large outside diameter than inside diameter and if the length can be equal to the length of a potted core on the same place, it can be roughly equal to the outside diameter. The temperature drop in the same material if. the same heat quantity is conducted, depends only on the geometric shape of the heat conductance pattern, characterized by the ratio of the length to the cross-section of the configuration in which the heat was conducted. This ratio is generally 7 to 10 in a potted core and /3 to A in a toroid, of the described dimension ratios. Hence the temperature difference between the hottest point and the outer surface of a toroid is only & to A of the same in the potted core; in actual examples 2.5 to 5 degrees centigrade. Such temperature differences are negligible in the system.

The heat transfer element can have many shapes. In FIG. 3, it is a tube having internally outstanding fins 36. The core 31 has now more turns of winding 32 and in each compartment formed by the finned heat transfer element 33 and the outer surface of core 31, two windings are placed. Naturally, it is possible to have the winding 32 in compartments 35 of the heat transfer element 33 in more than one layer. The heat transfer element 33 is glued to the ferrite core 31 by beryllia filled silicone resin and to the case 34 by soldering.

The windings 22 and 32 are both surrounded by air in the configurations of FIGS. 2 and 3, respectively, and consequently the parallel capacitance of the windings to the cases is small.

The heat transfer element could be made in one part together with the case, and then soldering of the same to the case would be eliminated. The gain in temperature difference is small, because very little temperature change occurs through the solder or adhesive layer.

In keeping with the present invention there are several additional embodiments which fulfill the requirements of the present invention. But in order to construct a device in accordance with the present invention, it should be noted that it is best that the windings of the toroid should not touch the cooling fins except in isolated spots where it can not be avoided. Since air is a very poor conductor of heat, it would appear that a much more effective fin pattern for the dissipation of the heat to the outer case would be constructed if the fin would cover the ferrite core which itself is a rather poor heat conductor. But if in doing this the windings were to rest on the portions of the fins covering the core, the parallel capacitance of the inductor to ground would be undesirably large since the thin insulating layer would be next to ground potential. This situation is avoided in the embodiments of FIGS. 2 and 3 where the fins do not touch the insulation of the wire wound around the toroid, only the magnetic core itself. Hence, the dominating part of the insulation between the wound wire and ground potential is air. Therefore the design considerations must take into account this air insulation along with the attempts to obtain maximum cooling.

It will be obvious to those skilled in the art that various changes may be made without departing from the spirit of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification but only as indicated in the appended claims.

What is claimed is:

1. An input inductor of a high power, high frequency filter comprising a magnetic toroidal core;

a toroidal coil on said core including a portion of the wire of said coil wound over the outer surface of said core;

a casing around the filter;

and heat transfer elements connected between said core and said casing and extending between winding of said coil without passing between said core and said coil windings;

whereby a low temperature drop path is formed between the outside surface of said core and the inside surface of said casing with no substantial increase of the parallel capacitance of the winding or the parallel capacitance between said winding and said case.

2. The input inductor of claim 1 further characterized by having a heat conductance pattern formed by said winding, magnetic core and said heat transfer elements, in combination having a large cross section and relatively short length of the heat transfer path between the hottest point of the inductor and the outer surface of the inductor.

3. The input inductor of claim 1 wherein said heat transfer elements are made of metal.

4. The input inductor of a high power, high frequency filter of claim 3 wherein said transfer elements are made of an alloy.

5. The input inductor of claim 1 further including a first thin adhesive layer connecting individual ones of said heat transfer elements to said core,

and a second thin adhesive layer connecting individual ones of said heat transfer elements to said casing.

6. The input inductor of claim 1 further characterized y said heat transfer elements connected to said magnetic core by an adhesive layer having a high heat conductive and electrically insulating filling.

7. The input inductor of claim 6 wherein said filling of said adhesive layer is beryllia.

8. The input inductor of claim 1 further characterized y an adhesive layer connecting individual ones of said 6 heat transfer elements to said casing having a high to said casing by an adhesive means having a high heat conductive metal powder filling. heat conductive metal filling; 9. The input inductor of claim 1 wherein and the substantially greater amount of the surface of said heat transfer elements are soldered to said casing. said wire wound over the outer surface of said mag- 10. The input inductor of claim 1 wherein 6 netic core surrounded by a gas; said heat transfer elements are integral with said whereby the parallel capacitance to said casing is mincasing. imum. 11. The input inductor of claim 1 wherein References Cited said heat transfer elements form an undulating shape UNITED STATES PATENTS posmoned around sald core 2 710 947 1955 Gaston 336 60 12. The input inductor of claim 1 wherein said heat transfer elements form inwardly directed 5/1961 Aheam et a1 336*60 XR 3,160,837 12/1964 Jones 336-61 fins mounted on a ring shape.

3,169,235 2/1965 Ouletta 33'661 XR 13. The input inductor of claim 1 wherein 8 6 the substantially greater amount of the surface of said 1965 Kates 317-100 wire wound over the outer surface of said magnetic ,744 3/1966 Halacsy 336*61 core is surrounded by gas, FOREIGN PATENTS whereby the parallel capacitance to said casing is min- 402 424 9/1924 Germany imum. 14. The input inductor of claim 1 further character- LEWIS MYERS primary Examiner ized by individual ones of said heat transfer elements connected THOMAS KOZMA, Amstam Emmmer' to said magnetic core by an adhesive layer having U S C1 XR a high conductive and electrically insulating filling; individual ones of said heat transfer elements connected 174-15; 33377 

